In many communications systems, specifically wireless networks, an information signal is modulated onto a higher frequency carrier before being transmitted. In these systems, there is often a need at the receiver for some form of frequency translation back to lower frequencies. A decision device is then used to recover the information.
In wireless systems in particular, there is a substantial amount of signal level variation at the receiver. Changes in the path loss, the distance from the transmitter, and other factors result in a wide range of signals which must be handled by the receiver. One of the most common methods of accommodating this high dynamic range requirement is with a variable gain element. This element (or elements), under the direction of some control circuitry, typically applies a variable gain to the received signal at some point in such a manner that the signal at the decision device is kept at a relatively constant amplitude.
Traditionally, the two functions of frequency translation and gain control have been separated into two distinct circuits. The frequency translation, a relatively simple process is usually done first with the more complex gain control being done at a lower intermediate frequency and often after a filtering stage to ease its implementation. There are several problems with this approach. The farther the gain control is from the receiver antenna, the higher the dynamic range requirements for all circuits before the gain control element including the input circuit of the gain control element itself. Thus there is an advantage to having the gain control function located close to the receive antenna. However, one of the biggest problems in the wireless arena involves receiving a small wanted signal in the presence of large out-of-band interferers. Since the filter stage mentioned above reduces the level of these interferers, it is easier to design the gain control function if it is located after the filter. As such, there is an advantage to having the gain control function located farther from the antenna. Thus the designer is faced with the decision to go with an implementation in which the gain control function is closer to the antenna which is more difficult to design and draws more power but eases the requirements of the rest of the circuitry or an implementation in which the gain control function is farther from the antenna after a filtering stage which is easier to design but places more burden on (i.e. increases the power consumption of) the preceding circuitry.
Gain controlled mixers have been implemented and/or proposed in the past but all previous implementations suffer from high noise and/or poor intermodulation performance (see for example Analog Devices, AD607 Low Power Mixer/AGC/RSSI 3 V Receiver IF Subsystem Datasheet, Rev O).